Distortion reducing circuits



an 13, 1942 F. H. SHEPARD, m 2.270.012

nI's'TRTIoN REDUCING CIRCUITS Original Filed March 51, 1936 2Sheets-Sheet 2 I IgI I P INVENTOR F. H. SHEPARD, JR. BY

ATTORNEY Patented Jan. 13, 1942 2,270,012 r DISTORTION REDUCING omcurrsFrancis H.

signor to Original application March 31,

Shepard, In, Merchantville, N. J., as- Radio Corporation of America, acorporation of Delaware 1936, Serial No.

71,941. Divided and this application March 2'7, 1940, Serial No. 326,180

11 Claims.

This application is a division of application Serial No. 71,941 filedMarch 31, 1936, now issued as Patent No. 2,198,464 on April 23, 1940.

The present invention relates broadly to relay circuits employingthermionic tubes and to such circuits as are incorporated in radiosignalling apparatus and the like.

In'the usual radio receiver, the detector output is fed to an amplifierand/or a power output circuit from which it is fed into a reproducerwhich may be a loudspeaker. Certain distortions are introduced due tothe characteristics of the amplifying and/or power output tubes of thesecircuits as well as due to irregularities in the output loading devicesuch as electrical and mechanical resonances of the output devices forinstance the loudspeaker.

The resonances mentioned above appear to be accentuated in practice whenthe output load is fed by a tube having normally high internalimpedance. The present invention contemplates reducing the apparentinternal impedance of the tube by feeding back to the tube input eitherall or a part of the A. C. output voltage in proper phase relationship.In this way the internal resistance of the tube becomes more effectiveto damp out the resonant peaks of the load, that is to say, the resonantpeaks of the load impedance are damped out by feeding back to the relayinput a predetermined percentage of the A. C. output voltage.

It is an object of the present invention to devise various circuitarrangements for overcoming the above mentioned distortions eitherinwhole or in part.

Broadly speaking, the invention contemplates methods of comparing a partor the whole of the amplifier output voltage with the input voltage andapplying any departure from a definite relationship between these, assignal, preferably to an intermediate amplifier in such a manner thatthe discrepancy will tend to be corrected.

The invention is particularly useful in connection with circuitsemploying a tube or tubes having a non-linear relationship between gridpotential and plate current or in circuits in which the tube output loadimpedance is not linear and/or not uniform at all frequencies.

Therefore instead of attempting to correct the load or tubecharacteristic it is contemplated in accordance with the invention tofeed back voltage into the grid circuit of the tube or tubes in such amanner that discrepancies between the input and output voltages create asignal which is applied to the amplifier in a sense to correct thediscrepancy.

According to the invention, the amplifier acts as a governor orregulator to control or keep the output voltage equal to a definitfunction of the input voltage. Any departure from this definite ratiocreates a signal which when fed into the amplifier creates currents inthe output of the amplifier which are of such magnitude and direction asto tend to correct the discrepancy.

The invention will be more readily understood by reference to thefollowing detailed specification when read in conjunction with thedrawings, the various figures of which illustrate various modificationsof the invention.

In the drawings:

Fig. 1 is a circuit representation of an arrangement which combines theadvantages of the circuit arrangements shown in Figs. 3 and '7 of theparent patent above referred to;

Fig. 2 is a diagrammatic illustration of an arrangement which combinesthe advantages of the arrangements shown in Figs. 5 and 7 of said parentpatent; I

Fig. 3 is a diagrammatic illustration of a modification of thearrangement shown in Fig. 1 of said parent patent wherein the differencebetween the input and output voltages is amplified before being appliedto the grid of the output tube;

Fig. 4 illustrates in diagrammatic form a variation of the arrangementshown in Fig. 3 in which the load can be placed in the plate circuit ofthe output tube; and,

Fig. 5 illustrates in diagrammatic form a phase inverter stage driving apush-pull output stage consisting of a pair-of output pentodes.

Referring to Fig. 1, tube is an output tube having a self-bias resistor40 by-passed for audio frequencies by the condenser 41. The A. C. outputvoltage of this tube is fed to the A. C. load through terminals 3 and 4.The choke 51 has a high A. C. impedance and is used to supply D. C.plate current to the tube '50. The grid of tube 50 is connected to thecathode of the tube 43. The plate of tube 43 is supplied with a positiveB voltage as shown and is effectively grounded for A. C. The cathode ofthis tube which is connected to the grid of tube 50 is returned toground through the high impedance inductance 31 and the self-biasresistor 38 which is bypassed by condenser 39. The screen of tube 43 isreturned for D. C. to the positive B voltage through inductance orimpedance 46, and it is grounded for A. C. to the cathode of tube 43 bymeans of condenser 45. The control grid of this tube is returned toground for D. C. through grid leak resistor 52. A. C. signal input isapplied between the plate of the output tube 50 and the grid of the tube43 by means of the input transformer 3. Condenser 49 serves as a meansof isolating the D. C. on the plate of tube 50 from the grid of tube 43.The input signal voltage is applied to the primary of transformer 3through the input terminals I and 2.

It will be seen on examining Figure 1 that the input voltage appliedbetween the plate of tube 50 and the grid of tube 43 must be ofsufficient value to overcome the A. C. signal on the plate of tube 50before any actual A. C. signal is applied between the grid and cathodeof tube 43. It can be seen that any discrepancy between the inputvoltage and output voltage results in a signal between the grid andcathode of tube 43. This grid to cathode signal tends to correct thedifference between input and output voltage. The arrangement shown inFig. 1 tends to maintain an output voltage across a load substantiallyequal to the input voltage. If the output voltage is across a lowimpedance load, however, it represents large power output compared tothe power input produced by the same voltage across the high impedanceinput circuit. Hence, although there may not be any amplification ofvoltage in the system there is a large and distortionless poweramplification.

Referring to Fig. 2, tube 50 is an output tube and tube 43 is a drivertube both of which operate in exactly the same manner as tubes 43 and 50described in Fig. 1. The difference of this circuit being that thesignal fed between the plate of tube 50 and the grid of tube 43 isgenerated in the plate circuit or across the plate circuit load 56 and51 of tube 53. Tube 53 is a high impedance pentode type of tube in whichthe plate current is only slightly affected by the plate voltage. Thecathode of tube 53 is returned to ground through self-bias resistor 65which is by-passed by the condenser 64. The screen of this tube 53 isreturned directly to the positive 13 voltage which is at an A. C. groundpotential. Signal input to this tube 53 is applied between the controlgrid and ground through terminals l and 2. This A. C. signal results ina variation of plate current which creates a varying voltage across theplate resistor or plate load 56 and 51. This voltage corresponds to thevoltage across the transformer 3 as described in Fig. 1.

It will be noted that the arrangement shown in Fig. 2 replaces the inputtransformer 3 of Fig. 1 by an auto-transformer 51 (see Fig. 2) connectedin the plate circuit of a preceding voltage amplifier tube. Insofar asthe feedback features of the invention are concerned, the arrangementsshown in Figs. 1 and 2 are substantially the same.

Fig. 3 is a multi-tube arrangement in which the A. C. input voltageacross terminals I and 2 is compared to the A. C. output voltage acrossterminals 3 and 4. The difference between the instantaneous values ofthe A. C. voltages results in a signal being applied between the gridand cathode of tube 60. The voltage output of tube 56 is amplified in aconventional manner by tube B1 and is applied to the grid of tube 68 insuch a manner that the output current of tube 68 is varied to buck outthe difference between the input voltage across terminals l and 2 andthe output voltage across terminals 3 and 4. By introducing a highdegree of amplification the discrepancy between input and outputvoltages can be greatly minimized.

In Fig. 4 tube 9| is an output tube the A. C. plate load of which may beconnected across terminals 3 and 4. The D. C. plate voltage may besupplied to the plate of the tube through the inductance 92. The bias tothe cathode of tube 9! is obtained by means of the self-bias resistor 88which is by-passed by the condenser 89. Tubes 80 and 30 act asamplifiers to amplify the voltage applied between the grid and cathodeof tube 80. This amplified voltage is applied to the grid of tube 9|.Potentiometer 04 is a high impedance voltage divider device placedacross the output load. The signal input voltage is applied to theprimary of transformer 3. The output of the secondary of transformer 3applies signal between the slider 93 of potentiometer 04 and the grid oftube 80. It can be seen that the difference between the voltage acrossthe secondary of transformer 3 and the voltage across the upper part ofpotentiometer 94 is applied as signal between the grid and cathode oftube (the cathode of tube 80 is held at A. C. ground potential by meansof by-pass condenser 83). It can be seen that it is not necessary tocompare the total output voltage with the input voltage but a part ofthe output voltage obtained by means of an inductance capacitance orresistance divider across the load can be compared to the signal inputas described above.

In Fig. 5 tubes 20! and 202 are connected in a conventional push-pulloutput stage. The grids of this push-pull output stage are drivenrespectively through condensers 201 and 208 by the plates of the doubletube 200. Input voltage fed to the grid of one triode section of tube200 results in an increased plate current in that particular section.This increased current through the inductor 209 causes the potential ofthe cathodes of tube 200 to change in such a manner that signal iscreated between the grid and cathode of the other triode unit of tube200 in such a manner that the current change through that triode sectionis almost equal and opposite to the plate current change of the firstsection. This effect results practically in equal and opposite voltagesbeing applied to the grids of the tubes 20] and 202. Part of the outputvoltage is obtained by the slider on pctentiometer 205. This voltage isapplied through condenser 206 to the grid of the second section of thetriode section of the duo tube 200. This signal is of such direction andmagnitude that it opposes the change created by the input signal to thegrid of the other triode section. In conclusion it can be seen thatsignal to the grids of the push-pull output stage is created only by adifference in potential between the two grids of the input of the duopurpose input tube. In this circuit the input voltage on the grid of thefirst triode unit of tube 200 is compared with a whole or part of theoutput voltage ob tained from the potentiometer 205 and applied to thegrid of the second triode unit of tube 200. This difference between thetwo grids of tube 200 results in a signal being applied to the grids ofthe output tube in such a manner as to correct for the difference inpotential between the two grids of tube 200. In other words, the secondgrid of tube 200 should have potentials equal to but opposite those onthe first grid of the tube.

I claim:

1. In an amplifier circuit a driven tube provided with an anode, acathode and a signal grid, connections external to said tube forming aninput circuit and an output circuit for said tube, a driver circuit forsaid tube including an electronic tube having an anode, a cathode and asignal grid, connections external to said second named tube forming aninput circuit and an output circuit therefor, means for coupling theoutput circuit of the driver tube to the input circuit of the driventube, a source of signal energy to be amplified, means for impressingthe energy to be amplified between the anode of the driven tube and thesignal grid of the driver tube and also between the anode and cathode ofthe driven tube, whereby the output voltage of the driven tube iscompared with the input signal energy voltage, a source of space currentand means including an output impedance device for connecting the anodeof the driven tube to the source of space current.

2. In amplifying apparatus, a driver tube having an anode, a cathode,and a signal grid electrode, a driven tube provided with an anode, a

cathode and a signal grid electrode, an input circuit for said firsttube including an input impedance, a condenser, a first outputimpedance,

a source of space current and a second output impedance connected inseries between the grid electrode of the first tube and the cathodethereof, an output circuit for said first tube comprising a connectionincluding the source of space current and said second output impedancein series between the anode and the cathode of the first tube, an inputcircuit for said driven tube comprising a connection including the saidsecond output impedance between the grid electrode and the cathode ofthe driven tube and an output circuit for said driven tube includingsaid first output impedance and the source of space current in seriesconnected between the anode of the driven tube and the cathode thereof..

3. In amplifying apparatus, a first electronic tube having at leastcathode, control grid and anode electrodes, an input circuit and anoutput circuit associated with said electrodes, an input impedanceincluded in said input circuit across which the energy to be amplifiedis impressed, an output impedance included in said output circuit, asecond electronic tube having at least cathode, control grid and anodeelectrodes, an input circuit and an output circuit associated with saidelectrodes, said latter input circuit including at least a portion ofsaid output impedance of the first tube whereby energy developed acrossthe output impedance is impressed upon the input circuit of said secondtube, an output impedance device included in the output circuit of thesec ond tube across which the output energy of said tube is impressed,and means for including at least a portion of said last named outputimpedance in said first named input circuit in series with said firstnamed input impedance whereby at least a part of the output energy ofsaid second tube is impressed upon said first named input circuit.

4. The arrangement described in claim 3 wherein at least a portion ofthe first named output impedance is included in the first named inputcircuit; in series with the first named input impedance and the portionof the second named output impedance which is included in said firstnamed input circuit, whereby at least a part of the output energydeveloped across the first named output impedance is impressed upon thefirst named input circuit.

5. In the arrangement described in claim 2, a screen grid electrode forsaid driver tube, means for applying a potential on said screen gridelectrode which is positive relative to the cathode, and means includinga condenser shunted between the screen grid electrode and the cathodefor maintaining a substantially constant difierence of potential betweenthe screen grid and the cathode.

6. In amplifying apparatus, a first electronic tube having a cathode, acontrol grid and an anode, an input circuit connected to the controlgrid including an input impedance, a condenser and at least a portion ofa first output impedance, and an output circuit connected to the anodeincluding a load resistor and a source of space current in series, asecond electronic tube having a cathode, a control grid and an anode, aninput circuit connected to the grid of the second tube coupled to saidoutput circuit of the first tube and an output circuit connected to theanode of the second tube including a load resistor and said source ofspace current in series, a third electronic tube having a cathode, acontrol grid and an anode, an input circuit connected to the grid; ofthe third tube coupled to the last named output circuit and an outputcircuit connected to the anode of the third tube including said firstnamed output impedance and said source of space current in series, thecathodes of the several tubes being connected to ground, and means forimpressing energy to be amplified across said input impedance includedin the input circuit of the first tube.

7. In an amplifier circuit, an electronic tube having an anode, acathode and a grid electrode, a circuit including a plurality ofimpedances in series connected between said anode and cathode, anauxiliary circuit including a source of space current for said tube anda plurality of impedances in series, connected between said anode andcathode, one of said latter impedances constituting the output impedanceof the amplifier, a second electronic tube having an anode, a cathodeand a grid electrode, a circuit including said source of space currentand an impedance device in series, connected between the anode andcathode of said second tube, means for connecting the grid electrode ofsaid second tube to a point of said first named circuit, means forconnecting the grid electrode of said first named tube to the cathode ofsaid second tube, a source of signal voltage, and means for injectingsaid signal voltage in said first named circuit between the grid of saidsecond tube and the anode of said first tube.

8. In signalling apparatus, a first electronic tube provided with ananode, a cathode, and a grid electrode, a connection between said an--ode and cathode including a source of direct current, a high impedanceinductance device and a bias resistor in series, a second electronictube having an anode, a cathode, and a control grid, means including ahigh alternating current impedance device for connecting said last namedanode to the anode of said first tube, said high impedance deviceserving as the output impedance of the apparatus, means including a biasresistor for connecting the cathode of said second tube to a point ofsaid first named connection intermediate said source of current and saidhigh impedance inductance device, a source of signal voltage, means forimpressing said signal voltage between the grid electrode of said firsttube and the anode of said second tube, means including an impedancedevice for connecting the grid electrode of said first tube to saidintermediate point of said first named connection, and means forconnecting the cathode of the first tube to the control grid of saidsecond tube.

9. In signalling apparatus, an output tube provided with an anode, acathode, and a control electrode, a driver tube provided with an anode,a cathode, and a control electrode, a source of signal voltage, animpedance across which said signal voltage is impressed, means includingsaid impedance and at least an additional impedance in series forconnecting the anode of said output tube to the cathode thereof, aconnection including a high impedance inductance and said additionalimpedance in series between the control electrode and the cathode ofsaid driver tube, means for connecting the cathode of the driver tube'to the control grid of the output tube, an output impedance connectedbetween the anode of the output tube and the anode of the driver tubeand a connection including a source of direct current and said highimpedance inductance in series between the anode and the cathode of saiddriver tube.

10. In signalling apparatus, a first electronic tube having an anode, acathode, and a grid electrode, a circuit including a load impedance, asource of space current and a biasing impedance all connected in seriesbetween the anode and the cathode of said tube, a connection including apair of impedances and said bias impedance in series between said anodeand cathode, a second electronic tube having an anode, a cathode, and agrid electrode, means including at least one of said pair of impedancesfor connecting the grid electrode of said second electronic tube to theanode of said first electronic tube, a source of signal voltage, meansfor impressing the signal voltage across said last named impedance,means connecting the cathode of said second electronic tube to thecontrol electrode of said first tube, means including the other of saidpair of impedances and the bias impedance in series for connecting thecontrol electrode of said second tube to the cathode of said first tube,a connection including the last named one of said pair of impedances andadditional impedance in series for connecting the control electrode ofsaid second tube to the cathode thereof, and means including said sourceof direct current for connecting a point of said last named connectionintermediate said other impedance and the additional impedance to theanode of said second tube.

11. The arrangement described in the next preceding claim wherein saidsource of signal voltage comprises a space discharge device having ananode, a cathode, and a grid electrode, a pair of signal inputterminals, means for connecting said grid electrode to one of saidterminals, means including impedance for connecting the cathode of saidspace discharge device to the other of said input terminals and meansincluding said impedance device which is connected between the gridelectrode of the second tube and the anode of the first tube, said loadimpedance, said source of direct current for connecting the anode ofsaid space discharge device to said other terminal.

FRANCIS I-I. SHEPARD, JR.

